Ebola virus is a single stranded negative sense RNA virus that belongs to the family Filoviridae and is a causative agent of viral hemorraghic fever (VHF). VHF viruses belong to the Filoviridae (which includes Marburg virus and Ebola virus), Arenaviridae (Lassa virus, Junin, Machupo, Sabia, and Guanarito viruses), Bunyaviridae (Crimean-Congo hemorrhagic fever virus, Rift Valley fever virus and Hanta virus), and Flaviviridae families.
There are five known species of Ebola virus: 1) Zaire Ebola virus; 2) Sudan Ebola virus; 3) Reston Ebola virus; 4) Tai: Forest virus, formerly the Cote d'Ivoire Ebola virus; and 5) Bundibugyo. Zaire Ebola virus is often considered the most virulent in humans. Although there are a number of species of Ebola virus the genomic structure of the filoviruses are very similar. The genome of the Ebola virus is approximately 19 kb in length and contains seven genes arranged sequentially in order from nucleoprotein (NP); viral protein 35(VP 35); VP40; glycoprotein (GP); VP 30; VP 24; and RNA polymerase (L).
Infection by VHF viruses results in rapid disease progression and high mortality rates. Infection by VHF viruses are associated with a wide spectrum of clinical manifestations, such as hemorrhage, diarrhea, myalgia, cough, headache, and pneumonia. Hemorrhage is the characteristic clinical manifestation of all VHF, although non-hemorrhagic infections are common. In the absence of bleeding or other organ manifestation it is often difficult to properly diagnose a VHF, and it can often be mistaken for other etiologic agents.
There are a number of currently available techniques to diagnose VHF and Ebola infections. These diagnostic assays include viral culture, immunohistochemistry, transmission electron microscopy, antigen ELISAs, and antibody ELISAs. However these techniques have a number of disadvantages which do not allow cheap, rapid, accurate and specific detection of Ebola viruses. For example viral culture is sensitive but highly specialized biological safety level 4 labs are required. Additionally viral cultures propagate the highly infectious virus. Transmission electron microscopy, although rapid, requires highly specialized lab equipment and trained personnel which precludes its use in the field for rapid detection. Antibody ELISAs are often unreliable because symptomatic patients fail to mount a detectable immune response prior to death. Antigen ELISAs provide rapid and accurate detection in the acute stages of infection but require the handling of highly infectious samples.
Reverse Transcriptase PCR is more sensitive than antigen ELISAs and has the added benefit of rendering the sample noninfectious prior to testing. RT-PCR can be easily deployed in the field to provide rapid and accurate testing. However, due to the conserved genomic sequence of Ebola and related VHF causative agents, prior RT-PCR assays often result in false positive results. Additionally prior RT-PCR assays are unable to reliably positively identify and detect all known subtypes of the Ebola virus.
The main route of transmission of VHF causative agents is contact with contaminated bodily fluids, such as direct contact with infected tissue or samples, close contact with infected patients, improper use of personal protective equipment or accidental exposure to contaminated laboratory equipment, such as, for example, needles. Because of the mode of transmission, high mortality rates, and need to follow and institute proper contamination controls and public health measures during an outbreak it is imperative to establish an early and accurate diagnosis. With a rapid and accurate diagnosis public health officials can institute appropriate case management procedures such as isolation measures or tracking of contact persons.
There is a present need for a test that provides sensitive, specific detection of VHF and Ebola viruses in a relatively short time and without the need of specialized laboratories, specialized training, or specialized equipment. Additionally there is a need for a rapid diagnosis that reduces autoinfection by laboratory technicians. Furthermore there is a need for a diagnostic assay that can be completed in sufficient time to permit effective treatment of the infected person and provide appropriate safeguards, such as isolation and contact surveillance.